The present invention relates to seals for use in a gas turbine engine. More particularly, the present invention relates to a hydro-lift seal for a circumferential seal segment which effectively reduces the rubbing forces experience by the seal thereby extending seal longevity.
In the aircraft industry, a "circumferential seal" is a name which describes a generic type of sealing device which is used in military and commercial aircraft. A circumferential seal is generally comprised of several arcuate segments made of carbon or other suitable material which are arranged circumferentially to form a continuous sealing ring around the periphery of a rotating shaft. The segments are typically supported against a race which interfaces the seal to a rotating element. The segment ends may contain overlapping tongue and socket joints to restrict leakage at the end gaps. These seals are used to separate areas of high pressure fluid from areas of lower pressure fluid. In a typical application, circumferential seals prevent the leakage of oil from a lower pressure compartment and minimize the flow rate of hot air from a high pressure area to the lower pressure, oil wetted compartment.
To achieve the long wear life which is demanded in modern engine applications, it is necessary to reduce the contact forces (unit loads) which interact between the stationary carbon segments of the circumferential seal and an adjacent surface of a rotating shaft or race connected to the shaft. In practice, the pressure drop between the high and low pressure areas reacts across the stationary carbon segments and generates a rubbing force between the segments and the adjacent rotating surface which is nearly proportional to the magnitude of the pressure drop.
Presently, state of the art circumferential seals are limited to pressure drops on the order of 20 to 40 psi for purposes of satisfying wear life requirements. In efforts to increase the pressure range of these seals, much work has been expended in the application of hydrodynamic gas bearings to the rubbing face of the carbon segments, i.e., to the face or bearing surface adjacent the rotating shaft.
Hydrodynamic gas bearings are well known devices within which a pressure rise is generated by the shearing gradient between the rotating shaft and stationary carbon elements. This pressure rise, acting against the bearing area on the surface of the seal segment, generates a force which is opposite in direction to the rubbing force generated by the ambient pressure drop. This effectively reduces the rubbing loads and increases the pressure range capability of the seal.
However, the problem with state of the art hydrodynamic gas bearings is that they require a very shallow depth. As presently configured and when used with gases as the medium, bearing pocket depths on the order of 0.001 inches or less are required to generate sufficient lift force to extend the pressure range of the seal. This inherent shallowness does not allow sufficient latitude to prevent wearing away the gas bearing during periods when surface speed is too low to generate sufficient hydrodynamic forces, or when centrifugal inertia, pressure and thermal gradients distort the rubbing interfaces and result in loss of gas bearing capacity. Therefore, when 0.001 inches of wear occurs, the gas bearings are utterly destroyed thereby creating an unacceptable situation. Therefore, a need exists for an assembly or apparatus which would effectively extend the allowable wear of a gas bearing.